Comprehensive optical performance monitoring in dense wavelength-division-multiplexed (DWDM) transmission systems offers the possibility of significant benefits, including real-time system optimization to improve performance, as well as fault isolation to reduce required repair time in the event of a failure.
In general, wavelength division multiplexing (WDM) is a technique which substantially increases the capacity of existing fiber optic networks. In a WDM system, plural optical channels are carried over a single waveguide (e.g., fiber), each channel being assigned a particular wavelength. Using optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical networks. Dense WDM (DWDM) refers to arrangements that utilize a relatively close wavelength spacing between adjacent channels. Current DWDM systems carry up to 160 channels spaced as closely as 50 GHz apart, with a channel power as low as −30 dBm (before being amplified).
Optical performance monitors (OPMs) in current DWDM transmission systems usually combine some type of wavelength-selective filtering device (such as a Fabry-Perot filter or a diffraction grating) with one or more conventional (slow) optical detectors. The OPM thus “tunes” through the various wavelengths with the filter and uses the detector to measure the power in each channel, as well as other quantities of interest such as the optical signal-to-noise ratio (OSNR). Various techniques have been developed in the art to improve the measuring capabilities of OPMs, particularly with regard to measuring OSNR. U.S. Pat. No. 5,986,782 issued to Alexander et al. on Nov. 16, 1999, discloses an arrangement that utilizes separate power meters for each wavelength so as to measure the OSNR for each channel by measuring the optical noise level at a wavelength near, yet separated from, each DWDM channel wavelength.
There remain various characteristics of an optical signal that cannot be measured with a conventional OPM, such as accumulated chromatic dispersion, polarization mode dispersion and the accumulation of in-band amplified spontaneous emission (ASE) noise within the bandwidth of a DWDM channel. The use of a sufficiently high-speed detector in an OPM would allow for these various characteristics to be measured, but the high cost (and is some cases, technical difficulty) of such a detector precludes the possibility of widespread use of such an OPM in commercial systems.
Thus, a need remains in the art for an accurate, yet economical, arrangement for enhancing the operation of an OPM to include the measurement of time-domain distortions such as chromatic dispersion, polarization mode dispersion and accumulated in-band ASE noise.